An A-B-A block copolymer that includes an amino acid block (A) that contains a polyamino acid and exhibits excellent biocompatibility, and a conjugated diene block (B) that contains a polydiene and improves physical properties (e.g., elongation at break) is known as a resin that may be used as a medical material and the like (see Patent Documents 1 and 2, for example).
Patent Documents 1 and 3 disclose a method for producing an A-B-A block copolymer that includes a polyamino acid block and a polyisoprene block. Specifically, a polyisoprene derivative that includes an amino group at each end is reacted with an amino acid-N-carboxy anhydride (e.g., γ-benzyl L-glutamate N-carboxyamino acid anhydride (BLG-NCA)) to expand each amino end of the polyisoprene derivative as a polyamino acid block. It is required that phosgene must be used as an initiator when synthesizing the amino acid-N-carboxy anhydride.
A polyisoprene derivative that includes an amino group at each end and is used as the raw material for such an A-B-A block copolymer may be produced by reacting a modifier (coupling agent) such as an anionic reactive compound (e.g., 2,2,5,5-tetramethyl-(3-bromopropyl)-aza-2,5-disilacyclopentane) that includes a protected amino group with the polymerization end of polyisoprene, and deprotecting the resulting product (see Patent Document 2, for example). Patent Document 2 discloses that the polyisoprene block can be produced by anionic polymerization using a difunctional anionic initiator obtained by reacting an alkyllithium with an aromatic hydrocarbon that includes two functional groups (e.g., 1,3-diisopropenylbenzene (DIPB)).
The reaction using the above coupling agent has a problem in that the amino group introduction rate (modification rate) of the end of polyisoprene is as low as 20 to 30%. Non-patent Document 1 discloses that an amino group can be introduced into the end of an isoprene block at an average introduction rate (modification rate) of about 90% by utilizing an imine compound such as N-(trimethylsilyl)benzaldimine as the modifier for introducing an amino group into the end of polyisoprene.    Patent Document 1: JP-A-5-317403    Patent Document 2: JP-A-7-80057    Patent Document 3: WO93/23453    Non-patent Document 1: Iwakazu Hattori et al., Makromol. Chem., 184, pp. 1355-1362 (1983)